Three-coordinate Co(I) provides access to unsaturated dihydrido-Co(III) and seven-coordinate Co(V).

The three-coordinate, T-shaped Co(I) complex, PNPCo (PNP = [(tBu2PCH2SiMe2)2N-], is readily synthesized by magnesium reduction of divalent PNPCoCl. Triplet (S = 1) PNPCo is coordinatively and electronically unsaturated and undergoes a thermally reversible oxidative addition reaction with H2, producing trivalent PNPCo(H)2. In contrast, the reaction with excess primary silane PhSiH3 quantitatively generates the base-stabilized silylene Co(V) compound {kappa2-tBu2PCH2Me2SiNSiMe2CH2tBu2P(H)Si=}Co(H)3(SiH2Ph)2.